1. Field of the Invention
The present invention relates to a variable capacitor. Particularly, it is concerned with a smaller-size capacitor being excellent in moisture proof property, heat-resistant property, uniform variability in capacitance, mass-producibility, quality-stability and abrasion-resistant characteristics.
2. Description of the Prior Art
The known conventional variable capacitor is usually constituted by sandwiching a dielectric material such as mica, ceramics, or a film of an organic polymeric material between a metal stator electrode and a rotor electrode.
A variable capacitor which employs mica as the dielectric material is excellent in the heat-resistant characteristics endurable for the soldering temperature but has a drawback in that it is inferior in the moisture proof characteristics, the quality-stability and the mass-producibility and thus disadvantageous in view of a desired miniaturization.
On the other hand, another kind of variable capacitor which employs ceramics as the dielectric material is likewise excellent in the heat-resistant characteristics, though it has drawbacks in the points of the mechanical strength, dimensional accuracy, mass-producibility and manufacturing cost.
Therefore, there is another variable capacitor which employs a film of an organic polymeric material as the dielectric material. The film of polymeric material may be polypropylene, polystryene, polyethylene or a fluorocarbon resin. A capacitor which employs such a film is however hard to provide the required dimensional stability because the film may expand or contract at the time of mass-production, and has other problems in the uniform variability in capacitance attributable to uneven adherence of film to both electrodes as well as in the moisture proof and heat-resistant characteristics.
Incidentally, another conventional type of variable capacitor which has at least one substrate metal electrode coated with a film of organic polymeric material by adhesion or fusion has become available.
As one example, there has been proposed a variable capacitor constructed by bonding polyethylene film over the surface of the electrode with adhesives, though it has not been put in practical use yet because of the expensiveness of the organic polymer film, the difficulty in even spreading of adhesives, the aberration of the film in the adhesion process, the expansion of the film by stretching, and the dimensional stability of the film itself.
Furthermore, there has been proposed a use of one which is made by roller-pressing the organic polymeric film on the metal electrode with or without heat-fusion or adhesives. Such an arrangment has, however, not been put to practical use yet beacause of the problems in the accuracy of the thickness of the organic polymeric film, the aberration of the film in the pressing process, the mass-producibility, the manufacturing cost and the like.